Final answer:
Slow-twitch muscle fibers have more mitochondria to support their role in endurance activities by maintaining energy production through aerobic metabolism over extended periods.
Step-by-step explanation:
Slow-twitch muscle fibers have more mitochondria because they are designed for endurance. These muscle fibers use aerobic metabolism to sustain contractions for extended periods. The mitochondria are crucial as they are the site where ATP is produced, which is needed for muscle activity. Fast-twitch muscle fibers, on the other hand, are designed for quick bursts of activity and contain fewer mitochondria. The presence of more mitochondria in slow-twitch fibers enables more aerobic metabolism, enhancing their capacity to maintain contractions over time, which is ideal for endurance exercises such as distance running or cycling.